(a) Field of the Invention
The present invention relates to a preparation method of a polypropylene and a polypropylene obtained therefrom. More specifically, it relates to a method of preparing a polypropylene by using a catalyst that includes a novel metallocene compound having an excellent polymerization activity, and a polypropylene obtained by the method.
This application claims priority to and the benefit of Korean Patent Application Nos. 10-2013-0116654 and 10-2014-0130844, filed in the Korean Intellectual Property Office on Sep. 30, 2013 and Sep. 30, 2014, respectively, the entire content of which is incorporated herein by reference.
(b) Description of the Related Art
Dow Co. had presented [Me2Si(Me4C5)NtBu]TiCl2 (Constrained-Geometry Catalyst, hereinafter ‘CGC’) in the early 1990's (U.S. Pat. No. 5,064,802), the superior aspects of the CGC to prior known metallocene catalysts in copolymerization reaction of ethylene and α-olefin can be largely summarized into two ways as follows: (1) it shows high activity even in high polymerization temperature and forms a polymer of high molecular weight, (2) the copolymerizing ability of α-olefin such as 1-hexene and 1-octene which have large steric hindrance is also very excellent. In addition, as various characteristics in the polymerization reaction of the CGC became gradually known, there have been many efforts to synthesize derivatives of the same for using it as a polymerization catalyst in the academic world and the industrial world.
Group 4 transition metal compound which has one or two cyclopentadienyl groups as the ligand can be used as a catalyst for olefin polymerization by activating the same with methylaluminoxane or a boron compound. Such catalyst shows unique characteristics that traditional Zeigler-Natta catalyst cannot realize.
Namely, the polymer obtained by using such catalyst has narrow molecular weight distribution and more good reactivity to the second monomer such as α-olefin or cycloolefin, and the second monomer distribution in the polymer is even. Furthermore, it is possible to control the stereoselectivity of the polymer in the polymerization of α-olefin by changing the substituent of the cyclopentadienyl ligand in the metallocene catalyst, and the degree of copolymerization, the molecular weight, and the distribution of the second monomer can be easily controlled in copolymerization of ethylene and other olefins.
Meanwhile, since the metallocene catalyst is more expensive than Zeigler-Natta catalyst, it must have good activity for its economic value.
As the results that many researchers have studied various catalysts, it is proved that generally a bridged catalyst has good reactivity to the second monomer. The bridged catalyst developed until now can be classified into three types according the type of the bridge. The first type is the catalyst of which two cyclopentadienyl ligands are connected with an alkylene dibridge by the reaction of an electrophile like an alkyl halide and indene or fluorene, the second type of the silicone-bridged catalyst of which the ligands are connected with —SiR2—, and the third type is the methylene-bridged catalyst which is obtained by the reaction of fulvene and indene or fluorene.
However, very few catalysts have been being applied in practice in commercial factories among above attempts, and thus the preparation of catalyst showing more improved polymerization performance is still required.
Furthermore, the metallocene compound is usually supported on a carrier when it is used to a gas phase or slurry process commerciallized in practice. At this time, it is attempted to load a cocatalyst together onto the carrier for securing the activity without using an additional cocatalyst in the polymerization process, but there is a disadvantage of that the heterogeneous catalyst supported on a carrier shows lower activity than the homogeneous catalyst that is not supported on a carrier in general.